Larval Fishes from Carrie Bow Cay, Belize in Color
Most marine fishes have some sort of pelagic larval stage that helps the species disperse from its place of residence to other suitable habitats a greater or lesser distance away. Some of these fishes have early-life-history (ELH) stages that resemble the adults and are simply smaller with somewhat different proportions and color patterns, for example, the pipefishes (Syngnathidae). Others, notably the eels, have larval stages so different in appearance that they were originally described as distinct genera and species, and in some cases even different families. In between these extremes lie every conceivable variation. Identifying larval fishes can be a severe challenge, since most of the characters that make the adults recognizable are absent. Nevertheless, until we can readily identify the larvae, we can do little to understand the life histories, migrations, and recruitment patterns of these fishes. The focus of our work at Carrie Bow Cay has been the identification and description of larval fishes.
Coral-reef fishes are famous for their brilliant color patterns, but most people are unaware that the larvae of these fishes are often quite colorful in their own right. When alive, these larvae are nearly transparent and have distinctive patterns of black, red, and yellow pigment. The black pigment (melanin) is the only one that remains after preservative, and the "pigmentation" given in most larval-fish descriptions refers almost exclusively to melanophores. The reds and yellows, which are sometimes quite prominent in freshly caught specimens, fade quickly in preservative and after a few weeks disappear without a trace. We have used two methods to document these colors. One is to add the antioxidant BHT (butylated hydroxytoluene) to the ethanol preservative, and the other is to photograph the freshly captured larvae. In this exhibit, we present photographs of a wide variety of larval fishes collected at Carrie Bow Cay. These photos represent a world that most of us never see, inhabited by a menagerie of tiny, glass-like creatures living their lives just beyond our view.
The Site: Carrie Bow Cay
Carrie Bow Cay is a small island on the edge of the world's second largest barrier reef, stretched in a north-south line off the coast of the Central American nation of Belize. There, since 1975, the National Museum of Natural History has operated a research station to study the complex biology of this extensive system of coral reefs and mangroves. These studies are under the auspices of the Caribbean Coral Reef Ecosystems program, Klaus Ruetzler, Director. The larval-fish program was initiated by G. D. Johnson in the 1980's. Baldwin and Smith joined the project in 1992. The photographs presented here were taken in August-September of 1996 and 1997.
Carrie Bow Cay is located just about 100 meters from the reef crest. A system of spur-and-groove coral reefs extends outward from the reef crest, deepening gradually and then rising in a secondary ridge a few hundred meters beyond, before plunging steeply to the floor of the Caribbean. On the landward side of the island is an extensive shallow lagoon studded with small islands, patch reefs, sand flats, seagrass beds, and mangroves, habitat for a multitude of fishes and other animals. Most of the larvae collected are in the later stages of their development and will soon settle out and transform into juveniles. The current flows consistently in over the reef crest and across the shallow flats in front of the island. A stationary plankton net hung off the dock on the seaward side will collect whatever is being carried in on these currents.
The water at the sampling site is about 0.3-0.5 m in depth. The collecting device was a 0.5 x 1 m rectangular plankton net with a mesh size of approximately 0.5 mm. It was suspended off the dock on the seaward side of the island and allowed to stream with the current. The height of the net's mouth was approximately equal to the water depth, so the entire water column was sampled. Sampling began shortly after sunset and usually continued until about 11:00 pm. The net was allowed to fish for 15-30 minutes, after which time the contents were emptied into a plastic sorting tray and brought into the laboratory. Individual larvae were removed with a small piece of plastic screening material and placed in a petri dish with sea water. A small amount of the anesthetic MS-222 was added to slow the fish down to allow observation. Selected specimens were photographed.
The camera used was an Olympus OM-1 with a 50-mm macro lens, attached to a bellows. The whole system was mounted vertically on a photo stand. The specimens were photographed in the petri dish, completely covered with sea water. A cover glass was usually placed over the specimen to prevent it from moving and to secure it in the proper attitude to be photographed. The dish was suspended about 8 cm above the surface of the table on a piece of plain window glass, supported by four plastic vials. This was necessary to avoid sharp shadows around the specimen. Illumination was provided by a single electronic flash, either hand held or secured to a second photo stand, positioned about 30 cm from the subject and at a 45º angle. Exposures were calibrated in advance and bracketed, generally three shots per specimen, one at the optimal setting, and one each at one stop above and one stop below. The film was Kodachrome 64. Two alternative backgrounds were used, one white or light gray, and the other black. The two give somewhat different results. The white background shows the chromatophores better, but the outline of the transparent fish sometimes gets lost. The black background works better for preserved, hence opaque, specimens.
The photographed specimens were preserved immediately afterward. They were initially fixed in 5% formalin in sea water. After remaining in formalin overnight, the specimens were transferred in stages to 75% ethanol to which BHT had been added. The BHT retards the fading of pigment and allows us to study the color patterns after returning to the museum. Some specimens were placed in holding tanks with running sea water. They were held for up to four or five weeks while they transformed. Many of these were also photographed.
David G. Smith, Ph.D.
National Museum of Natural History
Washington, DC 20560